This invention relates to migration-resistant binder compositions for bonding nonwoven fibers to form composites such as fabrics, bonded papers, paperboards, etc., the methods for applying these binder compositions and the nonwoven fibrous composites produced thereby. The bonded nonwoven fibrous composites may have a regular or a random array of fibers consisting of natural fibers, synthetic fibers, or mixtures of these. The bonded nonwoven fibrous composites are useful in the production of articles of various shapes, such as flat sheets or three-dimensional objects, and of various densities, such as low density insulating materials among others. Migration-resistant binders are particularly advantageous in the manufacture of nonwoven fabrics and saturated papers.
Binder migration in nonwoven composites is the phenomenon whereby binders that have been evenly distributed throughout the body of a nonwoven by a wet application, e.g., saturation, move to the outer face of the fabric during drying. Generally, this causes an increase in the fabric hand and a lack of strength in the direction normal to the plane of the sheet which, in extreme cases, can cause delamination of the treated web into two separate pieces. The alternative of increasing the amount of binder to achieve a specified strength would increase the cost of the product.
Binder migration is a consequence of the movement of liquid to the surface brought about by temperature differences between the surface and the interior of the webs. It is favored (1) by thick webs, (2) low binder/water ratios, (3) sudden, rapid heat increase, and (4) very stable latex binders. Efforts to control migration through the binder itself (i.e., as opposed to external changes in heating conditions, etc.) have generally centered on inhibiting binder movement after the binder is applied to the web through the use of (1) thickeners, e.g., alginates, or (2) coagulation agents which may be multivalent salts, e.g., aluminum sulfate, or cationic surfactants as noted below. The use of thickeners has several disadvantages, a principal one being that it limits processing speeds. The disadvantage of coagulating agents which work by decreasing the thermal stability of the latex is that the system must still be triggered by a temperature rise which then simultaneously favors binder migration, and the lowered degree of thermal stability increases the problems in handling the binder composition during plant processes such as pumping and mixing. In contrast, amine polymers of the instant invention do not decrease the thermal stability of the latex polymer with which they are used. Indeed, in the ordinary situation, the binder compositions are stable to 100.degree. C. Thus, because of both high shear stability and high thermal stability, our binder composition does not give trouble by breaking or flocculating if a pump through which it is passing, in a plant operation, warms it or if small amounts splash onto moving parts of the equipment, such as bearings.
In U.S. Pat. Nos. 2,912,349 and 2,912,350, Videen and others disclose impregnation of porous bodies with a heat-coagulable aqueous latex and, by applying heat, the coagulation of the latex solids within the pores of the body prior to volatilizing the residual water and thereby the prevention of migration of latex particles during drying. The two patents offer data to show that the coagulation temperature of the latex can be controlled by the addition of surfactants of ionic charge opposite to that of the latex, thus, that an anionic latex can have its heat coagulation temperature decreased by the addition of a cationic surfactant. The cationic surfactants recited include quaternary ammonium compounds and amines. In U.S. Pat. No. 2,982,682, Matlin and Kine teach a migration-resistant binder comprising a water-insoluble copolymer, having a molecular weight from 100,000 to ten million, and containing some amine groups and a water soluble aminoplast crosslinking agent for use in bonding nonwoven fibrous materials. In U.S. Pat. No. 3,300,429, Glavis et al. disclose a coating system which comprises a water-insoluble copolymer dispersion, a water-soluble ammonium salt of a low molecular weight acid copolymer, and a dispersing agent which is an anionic or a nonionic surfactant. At least one of the polymer components contains at least a quarter of a percent of polymerized units containing a ureido group one type of which includes those with amine functionality.
The migration of latex in saturated substrates during the drying process is frequently troublesome. Potentially, the distribution of latex in a masking tape or interlining non-woven substrate, for example, will be highly non-uniform if (1) a badly migrating latex is used, (2) no modifications are made in the manufacture of the saturated substrate, and (3) no external antimigration agents are used. Effects on the saturated paper or fabric properties can be postulated; one which is well-documented is the loss of delamination resistance which occurs when the latex distribution is such that a disproportionally low latex level is found in the center of the sheet. Such an effect can be compensated for by the use of a higher latex level or by changing one or more of the three factors enumerated above.
Such changes are not without other consequences, however. Some of these considerations are: (1) The selection of a non-migrating latex within the short list of those otherwise optimum in cost-performance benefits to the customer is usually not possible. Only a limited number of latices are significantly non-migrating and their cost-performance range is limited. (2) Modifications of machine running conditions, particularly the temperature of the first few drying cans or the use of an infrared heater bank in order to immobilize a thermally unstable polymer latex before the more efficient higher temperature drying sections are reached, are known to be effective to an extent. Further, any such adjustment would tend to require slowing the machine down if the production rate is drying capacity limited. (3) External anti-migration agents are available. An anionic latex can usually be immobilized at a lower temperature by the addition of a cationic water-soluble polymer. The resultant system frequently lacks sufficient shear stability to be used; in this case, non-ionic surfactant is post-added, apparently improving the mechanical stability without grossly affecting the thermal stability of the latex. Such systems are not only expensive, but require formulation by the converter. Further, the chemicals used frequently exhibit adverse effects on the color and heat-aged properties of the saturated sheets.